The major objective is to test the hypothesis that during the development of tolerance and physical dependence on morphine, certain interoceptive actions of morphine acquire secondary (conditioned) reinforcing properties as a result of their temporal contiguity with suppression of the morphine- abstinence syndrome that followed the last previous dose of morphine. It is further hypothesized that such secondary reinforcing properties of morphine persist long after subsidence of the unconditioned morphine-abstinence syndrome and facilitate relapse when the "postaddict" is exposed to the actions of morphine or other opioid for whatever reason. To test this hypothesis, rats will be trained to obtain water reinforcements in a 2-lever operant chamber and will then be divided into 3 groups: A, receiving continuous intraveous infusions of morphine for 6 weeks in gradually increasing concentrations to 200 mg/kg/day (never abstinent); B, receiving continuous intraveous infusions of normal saline for 6 weeks, with automated intravenous injections of morphine in gradually increasing doses to 200 milligrams/dg in a single dose each morning (abstinent every night); and C, receiving continuous intravenous infusions of normal saline with automated intravenous injections of normal saline according to the schedule in group B. At intervals of two weeks for 10 trial after abrupt termination of infusions and injections, all rats will be tested in the 2-lever operant chambers, one lever delivering reinforcements of the potent opioid, etonitazene (5 mcg/ml), and the other lever, water. In addition, the "protracted" abstinence syndrome will be compared in the 3 groups and residual tolerance to morphine will be compared in separate groups A, B and C, not trained in the operant chambers.